A cement retarder is a kind of a cement admixture (a cement additive) used for retardation of a hydration reaction of a cement, thereby retarding the setting or hardening of a mortar or concrete.
As disclosed in Japanese Patent Application Laid-open No. 172250/1989 (JP-A-1-172250), conventional cement retarders can be roughly classified into two categories, organic cement retarders and inorganic cement retarders. The inorganic retarders include lead oxide, boron oxide, borax, zinc chloride, zinc oxide, magnesium silicofluoride and the like. The organic retarders include sugar, a salt of an oxycarboxylic acid, a salt of lignosulfonic acid, gluconic acid or its salt, pyruvic acid, .alpha.-ketoglutaric acid and other keto acids and so on. These cement retarders are used mainly for longer hours of transportation of ready-mixed concrete in the summer season and for mitigation of stress due to a temperature (heat) when held in a large-sized concrete structure.
The surfaces of concrete products and various buildings are frequently decorated by means of washing (washing finish) technology of plastering technologies. The washing finish technology comprises washing a surface of a concrete product with water just before the hardening of a cement, so that the mortar on the concrete surface can be washed out, thereby exposing a part of aggregates on the surface. The washing operation has to be carried out in a strictly limited time period, because the timing for the washing operation is closely linked to the rate of the concrete hardening.
Concrete products produced by the precast technology are prevailing these days, and a wall member (a wall material), a concrete slab for pavement decoration and the like decorated by washing-finishing are manufactured in the factories. Regrettably, it is difficult to conduct a timely washing operation, although the rationalisation of production steps is of significant importance for this industry. Therefore, it is advantageous if the moulding step of the concrete slab and the washing step of the cement can be separated, so that the washing step can be operated irrespective of the hardening or setting degree of the cement.
By applying a cement-hardening retarder, which is a kind of a cement admixture (a cement additive), on the surface of a concrete product for retardation of the hardening of the concrete surface layer, the concrete surface can be washed out for decoration, regardless of the hardening rate of the cement. By way of illustration, Japanese Patent Application Laid-open No. 47725/1994 (JP-A-6-47725) suggests a process which comprises the steps of coating the inner surface of a mould with an aqueous solution containing a conventional cement retarder and a thickener in advance, depositing (casting) a concrete in the mould, removing the hardened article therefrom and washing out the surface of the article. Japanese Patent Application Laid-open No. 71412/1973 (JP-A-48-71412) and Japanese Patent Application Laid-open No. 141020/1979 (JP-A-54-141020) disclose the use of a conventional cement retarder which is impregnated in a sheet or paper.
The above cement retarders serve to retard cement hardening, the but effect is lost in a relatively short period. It is, therefore, difficult to improve the productivity of a concrete product with separating the washing step completely from the cement moulding step.